Center frequency stabilized frequency modulated oscillator



K. RATH March 2, 1948.

CENTER FREQUENCY STABILIZED FREQUENCY MODULATED OSCILLATOR Filed Feb. Y24, 1944 INVENTOR.

Patenied Mar. 2, 194s CENTER FREQUENCY STABILIZED FRE- QUEN CY MODULATEDOSCILLATOR I Karl Hath, New York, N. Y., assignor to Radio PatentsCorporation, New York, N.

poration of New York Application February 24, 1944, Serial No. 523,633

. 4 Claims. 1

My invention relates to high frequency oscillators of the type embodyinga piezo-electric crystal as a stabilizing means to obtain asubstantially constant oscillating frequency.

Crystal controlled oscillators known in the art utilize the crystal asan effective element of the oscillating circuit whereby the crystalforms an integral part of the oscillating mechanism. Ar-

ature.

Another disadvantage is the fact that a crystal controlled oscillatorcannot be directly frequency modulated, which necessitates the use ofphase modulation and other means to overcome this drawback. Furthermore,due to the extremely of the crystal, the starting or building up of theoscillationsv in many cases is not sufliciently smooth to enable rapidkeying and to suit other purposes and requirements.

Accordingly, an object of my invention is to provide a single tubecrystal controlled oscillator, wherein the crystal is arranged outsidethe oscillating circuit proper and serves merely as a stabilizing meansfor the oscillations generated independently of the crystal.

Another object is the provision of a single tube high frequencyoscillator circuit of the regenerative or feedback type comprising atuned oscillating circuit the frequency of which may be controlled inany desirable manner .and a piezo-electric crystal elementassociate'dwith the same tube or circuit and serving for stabilizing theoscillations or maintaining the carrier or center frequency of theoscillations at a substantially constant value.

These and further objects and advantagesof my invention will becomemore. apparent from the following detailed description taken inreference to the accompanying drawing forming part of this specificationand wherein:

Figure 1 shows an oscillator circuit embodying a piezo-electric crystalas a frequency control or stabilizing element in accordance with theprinciples of the invention; Figures 2 and 3 are graphs explanatory ofthe function and operation 'of the circuit according to Figure 1; Figure4 shows a Y., a cormodification of a single tube oscillator according tothe invention, and Figure 5 illustrates a complete frequency modulatedoscillator embodying a center frequency stabilized oscillator accordingto the invention. v

Like reference numerals identify like parts throughout the differentviews of the drawing.

Referring more particularly to Figure 1, I have shown at In anelectronic discharge tube comprising a heater element II, a cathode l2,an inner control grid l3, an accelerating or screen grid 14, an outercontrol grid I5 and an anode or plate 16, all substantially arranged inthe order named. A resonant oscillator or tank circuit, l1 is-;connectedto the inner control grid I3 and ground or cathode and coupled with theaccelerating grid [4 also known as an anode grid-through a feedback coill8 and blocking condenser- !9. A condenser-shunted resistance 20 isinserted in the common cathode return lead for the control grid andscreen grid circuits to provide proper grid operating bias potential ina manner well understood. The screen grid i4 is connected to thepositive pole of a suitable high tension or space current supply sourceindicated by the plus symbol in series with a load resistance 2 l Thecircuit connected to the cathode I2 and grids l3 and I4 constitutes astandard regenerative feedback oscillator capable of generating anoscillating frequency determined by the resonant frequency of the tankcircuit l1. Any other oscillator circuit known in the art andoperatively associated with the cathode l2 and grids l3 and I4 may beemployed for the purpose of this invention, as willbe readilyunderstood.In order to stabilize the oscillating frequency, I have shown apiezo-electric crystal 22 connected between the outer control grid !5and ground and shunted by a high impedance element. pervious to directcurrent such as a choke coil or high ohmic resistance 23 as shown inthedrawing. The plate I6 is connected to the positive pole of the spacecurrent supply source and by-passed to ground for high frequency currentby a condenser 24. The oscillations generated are applied to a suitableoutput or utilization circuit. connected to terminals 0-0 by way of acoupling condenser 25 or in any other manner known and well unfderstood.

It will be noted that the crystal 22 is connected in a circuit remote orsubstantially decoupled from the oscillator circuit proper andaccordingly will not be subject to the drawbacks'and defects of knownoscillator circuits, wherein the crystal forms an effective element orpart of the oscillator circuit. The function of the frequencystabilization in a circuit of the type shown in Figure 1 will be furtherunderstood from the following:

Electrons passing from the cathode l2 to the screen or accelerating gridl4 and subjected to a fluctuation in accordance with the oscillatingfrequency, will in part pass through the meshes of the grid l4 and beattracted and collected by the anode or plate It. In passing from thescreen [4 to the plate [6, the electrons will be decelerated due to theefiect of the negative electricfield surrounding the outer control gridl5 being at ground potential or having a potential negative with respectto ground due to the action of the biasing resistance 29.

As a result of this decelerating action, a dense electron cloud orconcentrated space charge also mal plate current Io. The average orsteady current passing to the screen grid [4 undergoes a similar changeas shown by the dotted line curve isg, that is, with the phase or senseof the variknown as a virtual cathode will be formed adjacent to thegrid l5, said space charge having a charge density which varies inaccordance with the oscillating frequency determined by the resonantfrequency of the tank circuit l1. As a result of this variable spacecharge, an oscillating current will be induced by electrostaticinduction in the outer circuit connected to the grid is and cathode l2and including the crystal 22. The phase of the induced current dependson the character of the impedence offered by the crystal andaccordingly, will be dependent upon the relative frequency departurebetween the oscillating frequency and the fixed resonating frequency ofthe crystal.

If, under normal conditions, the oscillating frequency or resonantfrequency of the tank circult l1 corresponds to the resonant frequencyof the crystal T2, the induced current passing through the crystal willbe exactly 90 out of phase with the oscillating frequency due to thefact that for this frequency the crystal offers pure resistive impedanceand that the induced current varies in accordance with the rate ofchange or derivative of the space charge fluctuations which latter arein phase with the oscillating frequency. If the oscillating frequencydeviates in either sense from the resonating frequency of the crystal22, the impedance offered by the crystal to the induced current willbecome either capacitative or inductive, resulting in a phase shift ofthe potential of grid l5 being greater or smaller, as the case may be,than the normal 90 or quadrature phase relation. Accordingly, thepotential upon the grid l5 caused by the drop of the induced currentthrough the crystal will have a phase with respect to the phase of thepotential on the grid l3 determined by the oscil ations, which variesboth in sense and magnitude depending on the departure of the.oscillating frequency from the resonating frequency of the crystal 22.

As a result of the dual control of the electron stream passing to theplate It by the potentials on the grids l3 and of varying relativephase, the average or steady plate or output current varies in a mannershown bythe curve is. in Figure 2 as a function of the frequency f ofthe oscillations, i. e. in turn the resonant frequency of the tankcircuit H. In case of equality between the resonanting frequencies ofthe tank circuit I1 and the crystal 22 the average plate current has avalue Io equal to the plate current if one of the grids 13 or l5 wereomitted or disconnected, i. e. determined solely by the steadyoperating, and bias potentials of the tube. This condition correspondsto a normal operating frequency identified as is in Figure 2. If theosciloperating potential of the screen grid 14.

ations reversed with respect to those of the anode current ia.

According to the present invention, the average plate or screen currentchanges as a function of an initial deviation of the oscillatingfrequency from its desired or normal value f0 determined by theresonating frequency of the crystal 22, are utilized to effect afrequency control to counteract the initial frequency deviation in amanner to restore the original or normal oscillating frequency. In theembodiment of the invention shown in Figure 1, there is inserted forthis purpose in the screen grid circuit a load impedance in the form ofan ohmic resistance 2| of suitable resistance value to produce apotential drop by the screen grid current isg causing a variation of thesteady screen grid potential. The latter reacts upon the tank circuit 11to vary the reflected reactance thereof in such a manner as to maintaina constant oscillating frequency independent of drift and other causestending to vary the resonating frequency of the tank circuit 11. Thisfunction will be further understood from the following.

It is well'known in vacuum tube circuits of this type that a reactance,usually a capacitative reactance, is reflected from the plate to thegrid due to inter-electrode coupling (Miller effect), which reflectedreactance is proportional to the output potential i. e. in the presentcase to the Since the latter varies in accordance with the deviations ofthe oscillating frequency from the predetermined resonating frequency ofthe crystal 22, the variations of the reflected reactance, by the properdesign of the circuit constants and parameters, will be such as tocompensate for or counteract an initial frequency variation in a mannerto restore and maintain a substantially constant oscillating frequency.

The reactance reflected into the grid or tank circuit I! usually of acapacitative nature, in some cases may be of an inductive type and mayvary in either sense in proportion to the potential on the screen gridI4. In order to obtain the proper phase of the reflected reactancevariations to counteract the initial changes of the oscillatingfrequency, various methods may be utilized depending on the particularoscillating circuit employed. Thus, in Figure 4 there is shown a circuitwherein the operating potential for the screen grid l4 serving as outputelectrode for the. oscillator is derived from the plate current is,having a phase opposite to the screen current changes as shown in Figure2. Alternatively, the crystal 22 may be utilized at either itsparallel-resonant or its series-resonant frequency, in which case againthe plate and screen current changes will be reversed. Figure 2corresponds to the operation of the crystal at its parallel-resonantfrequency, while Figure. 3 shows the plate and screen currents if thecrystal 2'2 is employed at its series-resonance. In the latter case,.asis seen, the phase of the plate and ing either screen grid or platecurrent changes for producing the operating potential for the screen l4,it will be possible to provide a screen grid potential varying in theproper sense as a result of an initial deviation of the oscillatingfrequency from its assigned value, whereby to counteract said deviationand to restore and maintain a constant oscillating frequency.

Referring to Figure 4, there is shown a circuit similar to Figure l witha somewhat different oscillator circuit connected to thev cathode l2 andgrids l3 and 14. This oscillator differs from Figure l in that theoscillating or tank circuit I1 is connected to the screen [4 and thefeedback coil I8 is connected to the control grid I3 in connection witha coupling condenser 21 and grid leak resistance 28. The plate circuitin this case includes a load impedance 30 to produce a plate potentialvarying in accordance with the relative frequency departure of theoscillating frequency from the assigned frequency of the crystal 22 asshown by the curve is. in Figures 2 and 3, depending on whether thecrystal is utilized at its parallel-resonant or at its series-resonantfrequency, respectively. The varying plate po tential serves forenergizing the screen grid or output electrode l4 of the oscillator;whereby to control the reflected reactance appearing in the grid circuitand affecting the resonant frequency of the tank circuit I! by virtue ofthe inductive coupling between the latter and the grid circuit.

Referring to Figure 5 I have shown a center frequency stabilizedoscillator similar to Figure 1 associated with a separate reactancecontrol tube for controlling the oscillating frequency in accordancewith the variations of a signal source such as a microphone or the liketo obtain a frequency modulated oscillation having a substantiallyconstant center or carrier frequency and an instantaneous frequencyvarying in proportion to the signal current variations.

There is connected for this purpose in parallel to the oscillator tankcircuit I! a reactance control tube 35 comprising a heater 36, a cathode31, a control grid 38, a screen grid 4! a suppressor grid 4| and ananode 42. The control grid 38 is excited by a quadrature potentialderived from the oscillating circuit by way of a phase shift networkcomprising a resistance 44 and a condenser 45 and connected across thetank circuit through a large blocking condenser 43. The screen grid40and anode 42 of the control tube are connected to a suitable spacecurrent supply source by way of potential drop resistors 46 and 47,respectively. The control grid 38 is furthermore excited by a signalpotential by way of an audio transformer 50 having a pri mary connectedto a battery 5| and a microphone or other signal device 52.

The function of the control tube 35 is well known. By varying thecontrol potential in accordance with the signals a variable reactancerepresented by the cathode-anode path of the tube is connected acrossthe tank circuit ll, resulting in a corresponding variation ormodulation of the oscillating frequency. In order to prevent potentialvariations of the anode [6 in accordance with the signal frequenciesfrom being impressed .upon the grid M, a choke coil 3| is insertedbetween the grid l4 and the load resistance 30 in the anode circuit ofthe oscillator. In this manner, only slow variations of the oscillatingfrequency due to drift and other causes will be impressed upon thescreen grid I4 and neutralized by the compensating or frequency controlaction 'in the manner described.

-If desirable, the'choke coil 3| or an equivalent linearity of themodulation, 'reducingnoise and obtaining other advantages of inversefeedback well known.

From the foregoing and the diagrams shown in Figures 2 and 3, it isfound, in the case of the standard triode oscillator associated with theelectrodes I2, l3 and I4 wherein the reflected reactance in the gridcircuit is of a capacitative nature in accordance with the well-knownMiller effect, that in order to counteract an initial o'scillatorfrequency deviation from the crystal frequency, the crystal should beoperated at series resonance when utilizing the screen grid current toproduce the frequency responsive oscillator anode voltage upon grid 43such as shown in Figure 1, and that the crystal should be operated atparallelresonance when utilizing the plate cur counteract this tendencyby reducing the oscillating frequency. This in turn necessitates anincrease of the oscillator anode potential upon grid l4 or, in otherwords, a decrease of the anode and screen current, whichever is used forproducing the frequency-responsive oscillator anode potential. 1 7

Hence, if the crystal is operated at its parallel-resonant frequency,corresponding to Figure 2, it will be the anode current inwhich'decreases with increasing oscillating frequency f, while if thecrystal is operated-at its series-resonant frequency, corresponding toFigure 3, it will be the screen grid current in which decreases withincreasing oscillating frequency.

Accordingly, in the case of Figure 1 where the varying screen'current isused to control the oscillator anode potential, the crystal 22 should beoperated at its series-resonant frequency, while in the case of Figure 5where the varying anode current is used to control the oscillator anodepotential, the crystal should be operated'at its parallel-resonantfrequency in order to counteract the initial oscillator frequencydeviations and to stabilize the oscillator.

Since both paralleland. series-resonant fre quencies of a crystal areclosely adjacent to each other, it may be easily determined, byexperiment and from the relationship shown in Figures 2 and 3, whichoperating mode should be chosen for a given oscillator circuit and agiven relation between the reflected reactance and the anode disclosedherein for illustration, but that the underlying thought and principleare susceptible of numerous modifications and variations coming withinthe broader scope and spirit of the in-- vention asdefined intheappended claims. The specification and drawing arev accordingly to beregarded in an illustrative rather thanin a limiting sense.

I claim:

1. An oscillator comprising, an electronv discharge tube having acathode, a first control grid, a screen grid, a second control grid andan anode, all arranged. substantially in the order named, a regenerativeoscillating circuit comprising a resonant tank circuit connected.between said first grid and said cathode and feedback coupling meanstherebetween and said screen grid to generate sustained electricaloscillations having a frequency determined by the resonant frequency ofsaid tank circuit, a piezo-electric crystal shunted by ahighimpedancepervious to direct current. and connected between said second controlgrid and said cathode, said second. control grid having a steady bias.to produce a concentrated electron space charge adjacent thereto andfluctuating at oscillating frequency, whereby to cause the steadyelectron current to vary in sense and magnitude in proportion torelative frequency departure between the generated oscillotions and theresonant frequency of said crystal, a load resistance connected to saidanode, means for applying varying operating'potential to said screengrid from said load resistance to thereby control, the reactancereflectedupon said tank circuit and to vary the oscillating frequency tocounteract initial deviations thereof from the crystal frequency, meansto modulate the frequency of the generated, oscillations in accordancewith a modulating signal amplitude, and means inthe screen grid circuitto prevent anode current variations in accordance with the modulatingsignal frequency from aifecting the steady potential of said screengrid.

2. An oscillator comprising an electron discharge tube having a cathode,a first control grid, a screen grid, a second control grid and an anode,all arranged substantially in the order named, a regenerativeoscillating system comprising a resonant. tank circuit connected betweensaid first control grid andv said cathode and feedback meanstherebetween and said screen grid to generate sustained oscillationshaving a frequency determined by the resonant frequency of said tankcircuit, a piezo-electriccrystal shunted by a high impedance pervious todirect current and connected between said second control grid and saidcathode, a load resistance connected to said anode, a conductive circuitconnection from said anode to said screen grid for applying varyingoperating potential thereto, an output circuit connected to said tankcircuit, further means. for controlling the frequency of the generated.oscillations in, accordance with a modulating signal, and means in thescreen grid circuit to substantially prevent anode current variations inaccordance with said modulating signal from aifecting the screen gridpotential.

. 3. An oscillator comprising an electron discharge tube having acathode, a first control grid, a, screen grid, a second control grid andan anode, all arranged substantially in the order named, a regenerativeoscillating system comprising a resonant tank circuit connected betweensaid first control grid and said cathode and feedback means therebetweenand said screen grid to generate sustained oscillations having afrequency determined by the resonant frequency of said tank circuit, apiezo-electric crystal shunted by a high ohmic resistance and connectedbetween said second control grid and said cathode, a load resistanceconnected to said anode, and means for applying varying operatingpotential to said screen grid from said load resistance, an outputcircuit connected to said tank circuit, further means for controllingthe frequency of the generated oscillations in accordance with amodulating signal, and audio frequency choke means inserted in thescreen grid circuit to substantially prevent anode current variations inaccordance with said modulating signal from affecting the screen gridpotential.

4. An oscillator comprising an electron discharge tube having a cathode,a first control grid,

a screen grid, a second control grid, and an anode, all arrangedsubstantialiy in the order named, a regenerative oscillating systemcomprising a resonant tank circuit and connected between said firstcontrol grid and said cathode, and feedback coupling means therebetweenand said screen grid to generate sustained electrical oscillationshaving a frequency determined by the resonant frequency of said tankcircuit, a piezo-electric crystal shunted by a high impedance perviousto direct current and connected between said second control grid andsaid cathode, a load resistance connected to said anode, means forapplying varying operating potential to said screen grid from said loadresistance, further means for controlling the frequency of the generatedoscillations in accordance with a modulating signal, an output circuitconnected to said tank circuit, and means in the screen grid circuit toprevent anode current variations in accordance with said modulatingsignal from affecting the screen grid potential.

KARL RA'IH.

REFERENCES orrnn The following references are of record in the file ofthis patent:

UNITED STATES PATENTS France NOV. 28, 1938

